Flomax is usually prescribed to be taken once a day in order to provide the most benefit. It's recommended that the drug be taken at the same time each day. To minimize side effects, a low dose is usually prescribed at first and then gradually increased. It may take up to 4 weeks to notice an improvement in symptoms.
As far as incontinence only while driving a car, this may be due to a change in or increased pressure on the abdomen (bladder, etc.) while sitting and moving the legs to operate the vehicle. For an accurate answer, however, this is a good question for your doctor.